It is to be understood that reference herein is made to the extrusion of a ply or a ply coat for a tire by way of example, as the invention applies to the extrusion of any profile. Modern passenger tires are typically constructed utilizing two or more layers of plies. Each ply usually includes a plurality of parallel reinforcement cords that are encased in a layer of an elastomer. Due to this structure, a standard ply has a rectangular cross section.
To manufacture a ply, an elastomer is processed by an extruder, and the processed or molten elastomer is fed or flowed to a crosshead die. The reinforcement cords are also fed into the crosshead die, where they are encased in the molten elastomer. A profile die is mounted on the exit portion of the crosshead die and includes a rectangular opening that sets the final cross-sectional shape and size of the extruded ply.
In the prior art, the flow path or channel inside the crosshead die for the molten elastomer has included a cross-sectional shape that is rectangular in order to approximate the shape of the opening of the profile die. Because the reinforcement cords must pass through the crosshead die in a straight, aligned manner, the flow channel for the molten elastomer is formed with a bend, which is usually 90 degrees. The combination of the rectangular cross section and the 90 degree bend creates a differential in the flow rate of the molten elastomer inside of the crosshead die, with the elastomer on the inside of the bend flowing more rapidly than the elastomer on the outside of the bend.
Such a differential in flow rate has led to non-uniform flow of the elastomer as it reaches the profile die, which results in a less-than optimum cross section for the ply. More particularly, due to the 90 degree bend, the actual flow of the molten elastomer tends to be centered at a point that is closer to the inside of the flow channel, rather than aligning with the center of the flow channel. It is desirable for the actual flow of an extruded ply to align with the center of the flow channel to create a uniform melt through the cross section of the ply, which optimizes the characteristics of the ply.
Therefore, it is desirable to provide a crosshead die for the extrusion of a profile, such as a ply coat for a tire, which includes a structure which aligns the actual flow of elastomer with the center of the flow channel to create a uniform melt through the cross section of the profile, thereby optimizing the characteristics of the extruded profile.